The objective of this study was to determine the rate of ultrasound-detected free-floating particles in amniotic fluid during the early second trimester and their relationship with maternal serum α-fetoprotein (MSAFP) in patients with normal amniotic fluid α-fetoprotein (AFAFP). Ninetyeight consecutive patients undergoing second trimester amniocentesis for various indications were prospectively studied. Before undergoing amniocentesis, each patient had a level II ultrasound examination and evaluation of the presence of free floating particles. A subjective estimate of the particle amount and measurement of the size of the largest particle seen were made. Patients were stratified into three groups according to their MSAFP level (low, normal, high). Statistical significance of results was assessed by analysis of variance and multiple comparison procedure, and by nonparametric procedures, as appropriate. MSAFPs (mean ± 1 SD) were 0.41 ± 0.18 and 4.88 ± 2.22 multiples of the median for the low and the high groups, respectively. All AFAFPs were within normal limits. Ninety four percent of patients with high MSAFP had free-floating particles in amniotic fluid as compared to 43% in the low and normal groups (p < 0.01). Patients with high MSAFP had significantly greater density and size of particles. The presence of ultrasound-detected free-floating particles in amniotic fluid of normal patients during the early second trimester may preclude its use as a reliable indicator for fetal lung maturity, or suggest that the source of these particles may differ by trimester. High MSAFP is significantly correlated hith the ultrasonographic appearance of free-floating particles, as well as with larger size and higher am